Stainless steel having improved machinability

ABSTRACT

1. A METHOD OF MAKING STAINLESS STEEL OF A MARTENSITIC GRADE TO HAVE IMPROVED MACHINABILITY, COMPRISING ESTABILISHING A SOLIDIFIED STAINLESS STEEL WHICH IS COMPOSITIONALLY CAPABLE OF TRANSFORMATION TO MARTENSITE AND WHICH CONTAINS GLOBULAR INCLUSIONS THAT ARE DISTRIBUTED THEREIN AND THAT COMPRISE MN, S AND ELEMENT OR ELEMENTS SELECTED FROM THE CLASS CONSISTING OF SELENIUM AND TELLURIUM, CONVERTING SAID STEEL TO A SHAPED OBJECT, AND THEREAFTER TREATING SAID SHAPED OBJECT TO CHARACTERIZE THE STEEL THEREOF AS CAPABLE OF YIELDING SHORT-BREAKING CHIPS ON MACHINING, BY HEATING THE OBJECT TO A TEMPERATURE BETWEEN THE LOWER AND UPPER CRITICAL POINTS OF THE STEEL, FOR A PERIOD OF AT LEAST ONE HOUR, AND THEREUPON COOLING THE OBJECT.

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INVENTOR.

V|$AYAK MAR lPNIS W S- (9 mm 1974 v. A. TlPNIS STAINLESS STEEL HAVINGIMPROVED MACHTNABILITY Original Filed April 6, 1970 6 Sheets-Sheet 4 6001000 p/ECES MACH/NED INVENTOR. V! J AYA KUMAR A. HP/W5 5, 1974 v. A.TIPNIS STAINLESS STEEL HAVING IMPROVED MACHINABILITY 6 Sheets-Sheet 6'Original Filed April 6, 1970 N0 Te Pl ECES MACH/NED iqlE.

INVENTOR. H AYAKUMAR A. Twms W S.M

United States Patent O US. Cl. 148135 6 Claims ABSTRACT OF THEDISCLOSURE Improved machinability of stainless steels, as determined byproduction-type operation of automatic screw machines, is obtained: byproviding globular inclusions in the steel, which comprise essentiallysulfur and selenium or tellurium, in combination with manganese, andwhich maintain globular character through extensive reduction as by hotrolling, the proportions of such elements being controlled to producethe inclusions, very preferably with a specific, relatively low andeconomical addition of selenium or tellurium; and also by controllingthe actual presence of aluminium oxide in the steel, including thecontrol of deoxidation practice, to provide production of stainlesssteel having not more than a critically very low content of aluminumoxide. In martensitic grades, the improved stainless steels are furtherenhanced in machinability, eg as to chip characteristics, by specialheat treatment, including heating between the A and A critical points oftemperature. Optional additions of rare earth elements can coact inestablishing or enhancing the desired, e.g. sulfide-selenide,inclusions, and can afford deoxidation function effective in avoidingunwanted aluminum addition, for corresponding cooperation in minimizingoccurrence of aluminum oxide.

This application is a division of application Ser. No. 26,009, filedApr. 6, 1970, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to stainless steeland is more particularly directed to the provision of stainless steelshaving superior machinability, and to methods of producing such steels.

Although much work has been done toward the development of so-calledfree-machining stainless steel, and although steels so classified havebeen marketed with some success for a number of years, the machiningproperties have tended to fall short of those attainable in plain carbonsteels designed for good machinability, and there appears to have been afailure of understanding or full appreciation of the nature of theproblems presented by stainless steel in this area. Indeed, requirementsand conditions can be considered more critical than in the low or mediumcarbon grades of ordinary steel, in that: machined products of stainlesssteel, which is more costly, are often expected to have a much betterfinish and dimensional control than carbon steel products,'whilesecondary operations such as reaming, threading, tapping and grinding onstainless grades can be very uneconomical at high volumes of production,as in shops using automatic screw machines; and stainless steels areinherently unsatisfactory to machine not only because of general cuttingdifficulties but especially because their thermal diffusivity is abouttwo-thirds to one-half that of plain carbon steel, causing higher toolchip temperature and consequently shorter tool life.

Particular stainless steels that have heretofore been produced withspecial characteristics of machinability include AISI type 303 of theaustenitic 300 series, and A151 type 'ice 416 from the martensiticgrades of the 400 series, as well as others, e.g. the ferritic type430F. Athough the principles of the present invention are primarilyillustrated with the so-called free-machining varieties such as the 303and 416 stainless steels, the same principles are deemed applicable to abroad range of stanless steels embracing the 200, 300 and 400 series,and also the precipitation hardening grades of both the martensitic andsemi-austenitic types, such broader range being inclusive of steels forwhich the end service properties might restrict the usages offree-machining additive to low levels. As explained below, the inventionhas been developed with special applicability to the presentlyrecognized machining grades, for exam le types 303 and 416, andcorresponding particular objects are to provide improvement in suchgrades, notably in suiting the properties of these steels to therequirements of machining operations in industrial production.

It has been known that the addition or intentional inclusion of one ormore elements such as sulfur, selenium, tellurium, lead and bismuth canbe beneficial to machining properties in essentially all kinds of steel,and such additions of sulfur or selenium, for instance, in the amountsof 0.15% or more have been employed in stainless types 303, 416 and 430Fto provide the basis for rating such types as machinable. In a producedsteel sulfur additions usually appear as sulfide inclusions, basicallyas manganese sulfide and these inclusions can exhibit a variety ofmorphology and may contain one or more other elements, such as theso-called transition elements, dissolved therein, all depending on avariety of compositional and processing factors that have not heretoforebeen fully elucidated.

Various and not entirely consistent views have been expressed as to thecircumstances for enhancement of machinability by the use of sulfur orfor assurance of a supposedly effective kind of inclusion. Thus forexample, some investigators have proposed, chiefly on the basis of theratio of manganese to sulfur in the steel (i.e. between the totalamounts of these elements, in weight percent), that improvement inmachinability of stainless steels can be effected by reliance on asupposedly proper range of Mn to S ratios alone. In other cases, one oranother of various considerations, chiefly of the nature of specificadditions or omissions of chemisty, have been asserted to be useful, butthe extended investigations upon which the present invention ispredicated have revealed that there has heretofore been a failure tounderstand or recognize many underlying factors, bearing on the natureand effectiveness of sulfide inclusions, or what inclusions are trulysuitable for achieving machinability or how they can be achieved,notably in production type quantities of steel, or what may be theeffect or significance of steelmaking processes on machinability and onadditions supposed to improve it.

It appears, moreover, that past studies have in general failed to takeproper account of various requirements of machinability, particularlythe needs of industry in making machined articles from stainless steel.Thus in some cases sole reliance has been placed on limited drillingtests, such for example as in measuring the time required for a givenpenetration by a specified drill under a constant load, but thesedeterminations have given little or no indication of performance inregard to surface finish, tool wear, tool life, or chip characteristics,or indeed in practical productivity, e.g. the speeds and feeds that canbe used in production machinery. Attention has, of course, been given toone or another of these factors in other discussions of free machiningsteel, but their collective significance has not been emphasized. Moreimportantly, in the testing or design of new steel compositions thereappears to have been essentially no recognition, and certainly no reportof systematic use, of production-type studies such as involve, forinstance, a continuous run of several hours of an automatic screwmachine producing 1000 to 1500 or more pieces of the test steel, eachsubjected to a machining cycle which includes major machining operationsthat reveal the performance factors mentioned above and which isrepresentative of the kind of work required by industrial users.

Definitive information on essentially all factors of machiningperformance is obtainable with productionsimulating tests of this sort,but they can be usefully supplemented or extended by specificsingle-purpose tests of rigorous design, such as turning tests for toollife and tool wear, plunge cutting for surface finish determinations,and drilling tests using suitably large drills for chip breakabilitydeterminations. Investigation has indicated that selection ofcompositional ranges and other characteristics to provide supposedlymachinable steels, based only on one or a few limited tests such asdrillability used by many of the previous investigators, can be veryunreliable, and in particular do not afford good correlation with trueand complete requirements for machining stainless steel, i.e.requirements as outlined above that must be met in industrial practice.

Accordingly, important aims of the present invention are to affordimproved stainless steels, and methods of producing them, which havedistinctly superior machining properties and which in presentlypreferred embodiments are well suited to the needs, in quality andproduction rate, of manufacturers of machined products. A further objectis to provide such steels and such. methods in an economical manner, andwithout significantly altering the other desired properties thatcharacterize the grade of steel to which the invention is applied.

SUMMARY OF THE INVENTION To the above and other ends, important aspectsof the invention reside in the discovery that superior machiningproperties in stainless steels are not only dependent on the presence ofinclusions which are of the general nature of those heretofore classedas manganese sulfide inclusions and which are of a so-called globulartype, but are also dependent on the volume fraction and the shape,distributional, compositional and mechanical characteristics of suchinclusions, and on the existence of these characteristics after thesteel has been carried through the usual production operations such ashot rolling. These requirements cannot be assured, and indeed usuallyfail to be realized, in reliance simply upon a factor such as a socalledmanganese-sulfur ratio, determined as the proportion of total manganeseto total sulfur in the steel. On the contrary, it is important that theinclusions be present in such kind, amount and size as has beendiscovered to be correlated with the more complete and significant testsof machinability explained above, and as has specifically beendiscovered to involve further or additional compositional and processingfactors, not heretofore recognized.

Thus a significant feature of the invention, in its specific andpreferred aspects, resides in the finding that instead of relying onsulfur as the sole addition to promote machining properties, improvedcharacteristics of the inclusions are attained by incorporation ofselenium, or in some cases alternatively or additionally by theincorporation of tellurium. Particular advantage, directly related toimprovement in tool life and surface finish in machining, is provided bycompositions containing both sulfur and selenium, Where the selenium ispresent in amounts that may be significantly and very desirably lessthan the quantities usually specified, e.g. in AISI types 303 Se, 416 Seand 4301 Se, for attainment of free machining. Specifically compositionswhich thus contain sulfur, and selenium in amounts less than 0.15%,preferably 0.04 to 0.1%, together with manganese in sufficient amountsto satisfy the theoretical stoichiometric requirements of MnS and MnSeas well as to account at least for other unavoidable or requiredutilization of manganese in the steel (such as constituting a milddeoxidizer and a matrix strengthener), are found to provide inclusionsof superior and assured characteristics for machinability, notheretofore or reliably achieved with inclusions predicated on sulfuraddition alone. At the same time, the steel is relatively economical toproduce, for example as compared with the special grades last mentionedor as measured, in effect, against the attained improvement inmachinability.

Further features of the invention are based on the finding thataluminum, even in amounts heretofore considered inconsequential, as forexample the small quantities conventionally used for deoxidizing (i.e.killing), and indeed even in smaller amounts that would ordinarily bedeemed incidental, may lead to detriment in machinability, specificallyto the extent that the aluminum becomes or appears as aluminum oxide,i.e. alumina, A1 0 Extended studies involving analysis of steels foraluminum oxide content, which is not ordinarily determined or which isordinarily considered of no consequence at the levels so studied, haverevealed that tool wear and tool life in machining are very sensitive toaluminum oxide in the steel; for example, Whereas an ordinarycommercially produced, free machining grade 416 may show 0.006% or moreof A1 0 i.e. an amount not usually deemed consequential or even commonlymeasured, limitation of such oxide content to 0.02% and below hasafforded large increases in useful tool life, of the order of 50% to ormore. The aluminum oxide appears as a distribution of minute, hardparticles or inclusions, further evidence being that they tend to showup in manganese-sulfur inclusions and apparently even influence the verynature of precipitation of the latter in an adverse manner, as bypromoting socalled entectic or type II sulfides which become long,stringy configurations of the manganese'sulfur bodies in as-rolled steelbars.

Thus the invention, in one related aspect, consists in steel of thestated character wherein alumina is kept to an unusually low maximum,such as 0.0025% or more advantageously 0.002%, and is preferably wellbelow such values, a further feature being that the process of makingthe steel involves deoxidation otherwise than by the use of aluminum, asfor instance by employing silicon (conveniently in the form offerro-silicon), and indeed more specifically by using such agent in aform having no more than a very low impurity content of aluminum.Another aspect of the invention is that the incorporation of seleniumalong with sulfur, in the manner explained above, has been demonstratedto reduce materially the tooldestructive effect of aluminum oxide, itbeing further noted that at moderately small levels of A1 0 (yet above0.002%) the manganese-sulfur-selenium inclusions retain their desiredsubstantially globular shape and appearance, and have been observed asfunctioning, at least in part, to provide a sheath or enclosure for thealuminum oxide particles.

As a supplemental feature of the invention, constituting an addition orin part alternative to manganese for the composition of thesulfide-selenide inclusions, rare earth metals such as lanthanum, ceriumand others may be employed, with good effect on the machinability of thesteel in one or more of the respects of tool life, surface finish, easeof chip removal, and productivity. These elements can form sulfides andselenides, or possibly complex compounds of such nature with manganese,and produce the desired globular inclusions or appear in them, impartingcharacteristics that are similar to those afforded by compounds ofmanganese with sulfur or selenium. A further procedural feature isafforded by the step of adding a rare earth metal or metals, as at theend of a heat or in the ladle, in that such addition may serve thedeoxidizing function in lieu of silicon or other substitute foraluminum, and then at least in part the rare earth addition may appearas sulfide of selenide compounds in the inclusions or some of them,promoting formation of such inclusions in the desired maner. Suchresults are attainable, for example, by adding two to three pounds perton, of a rare earth alloy of common type, containing predominantlylanthanum and cerium, with minor proportions of others.

It has also been found that martensitic grades of stainless steelproduced to contain the desired, unusually effective inclusions, e.g.comprising manganese with sulfur and selenium, may be further benefitedby a special heat treatment, particularly in that the chip formation,for various kinds of machining operations, can be greatly improved. Thatis to say, in some cases even with optimum form and volume fraction ofthe sulfide-type inclusions, machining chips may in fact be very long,tough ribbons or curls of ditfculty manageable type. In accordance withthis further feature of the invention the improved steel, as forinstance of the 416 grade, is subjected to heat treatment which includesheating to a temperature between the lower and upper critical points,i.e. between the A and A temperatures for the given composition, holdingthe piece at the temperature in an inert atmosphere furnace for one andone-half hours or more, depending on the diameter of the bar or othershape, and then cooling in air to room temperature. If desired, thearticle can thereafter be tempered at a suitable, lower temperature, andalso stress-relieved. In circumstances Where the cutting or drillingchips may tend to be several feet long without this treatment, itsefiect is to cause the chips to break off short, e. g. at a few inchesor less. It is believed that the treatment, notably if performed in apreferred manner as explained hereinbelow, results in a two-phasemicrostructure, partly martensitic and partly ferritic with carbides.

The effect of practice of the invention in one or more of its aspects,and preferably in respect to the controlled addition of selenium incoaction with the essentially complete elimination of alumina, forattainment of optimum nature and properties of the described inclusions,including the distinct and critically advantageous characteristic thatsuch inclusions are not materially altered in their globular orellipsoidal shape, or in particular, fiattened to long, thinconfigurations, by hot rolling, has been to achieve improvement of avery practical sort in the machining properties of stainless steels.These and other advantages of the invention, and additional disclosureand explanation of various compositional and procedural featuresthereof, are also set forth or will become further apparent in thefollowing detailed description, including reference to specific heatsand practices by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1, 2 and 3 are microphotographicviews respectively showing three different types of sulfide inclusionsin steel.

FIG. 4 is a microphotographic view showing inclusions, of the generaltype of FIG. 1, as appearing in a stainless steel embodying theinvention.

FIG. 5 is a view, to be compared with FIG. 4, showing undesirablesulfide inclusions.

FIG. 6 is a graph illustrating the distribution of plane interceptdimensions corresponding to distribution of inclusions in an example ofone type of stainless steel embodying the invention.

FIG. 7 is a graph, like FIG. 6, showing such interceptdimensiondistribution for another type of stainless steel of the invention.

FIG. 8 is a microphotographic view, comparable with FIG. 4, showingaluminum oxide particles in a stainless steel.

FIG. 9 is another view, on further magnified scale, of inclusions in anexample of the invention.

FIG. 10 is a view, to be compared with FIGS. 8 and 9, showing aluminumoxide particles in a stainless steel embodying certain features of theinvention.

FIG. 11 is a graph of the results of screw machine tests 6 of stainlesssteels, showing the effect of aluminum oxide in the steel.

FIG. 12 is a graph of the results of screw machine tests, comparativelyshowing the effect of aluminum oxide in stainless steel without and withselenium in accordance with the invention, and in stainless steelproduced in accordance with a further feature of the invention.

FIG. 13 is a graph of the results of screw machine tests, showing theeffect of tellurium addition in accordance with the invention.

DETAILED DESCRIPTION As indicated, the investigations leading to thepresent invention and the evaluations of stated results were largelybased on production-type tests of long duration. Specifically, each testconsisted of an 8-hour run on an automatic screw machine, supplied withl-inch diameter bars and automatically turning out small finishedpieces, usually a total of 1,500 to 1,700 pieces for the 8-hour run. Thegeneral practice in preparing the test bars involved hot rolling from4-inch billets, followed by a small amount of cold reduction (when thespecial heat treatment of the present invention was used, it wasperformed after the cold reduction), the bar being thereafter turned andground, in other equipment, to have the finished 1- inch dimension.Utilizing a 6-spindle automatic screw machine, it was charged with sixbars, each 12 feet long, and in general each 8-hour run required threeto four further charges depending on the cutting speed and feed chosenfor the test.

Various combinations of machining operations, generally using standard,hardened, high speed steel tools, were performed on each piece, atypical setup involving rough forming, finish forming, facing, threedrilling operations, reaming, and cut-off. In some instances additionalor alternative finish machining operations were used, such as shaving,thread cutting and double reaming. The machined pieces were cut off atabout 1 /2 inch lengths. Runs of this sort permitted evaluation ofnumerous aspects of machining practice, with respect to the particularstainless steel under test, i.e. the selected heat from which theprepared stock of l-inch bars was obtained.

A notably important measurement was with respect to surface finish, i.e.on the outside surface of revolution of the test pieces, resulting fromthe finish forming step. This surface finish measurement was made with aBrush surface analyzer, yielding a roughness determination,corresponding to the wave height or peaks of surface roughness. As willbe understood, the measurement was thus determined, conventionally, asmicroinch R.M.S. (root-mean-square), one minimum criterion being toachieve a surface roughness less than microinches R.M.S. over a toollife of at least the stated 8 hours at a chosen combination of feed andcutting speed. The cutting speed, as will also be understood,represented the relative speed of the surface of the work, i.e. in thecircumferential direction, past the cutting tool; for machining gradessuch as 303 and 416 the selected speed was surface feet per minute orhigher. The end slide feed of the machine (i.e. the drill feed) was alsomaintained at a desirable production value, for instance of the order of0.005 inches per revolution for grades 303 and 416 and the like. Thecross slide feed was about one-seventh of the end slide feed. The finishforming operation was usually a plunge cut, employing a tool ofappropriate width moved radially inward of the workpiece and therebyproducing an annular machined area which has a width equal to that ofthe tool and is intended to be characterized by a finished surface.

At regular intervals, finished pieces were removed for surfacemeasurement and other examination. Standardized practice involved takingsix pieces at every 100-piece interval and examining each with a surfaceanalyzer. Determinations were plotted, against duration of the test,

by taking an average of the surface roughness for each set of sixpieces, and also delineating the range of varia tion among the six. Ingeneral, a single tool was allowed to remain in place, withoutsharpening, throughout the entire 8-hour run. In such operation, thegradual rise in surface roughness and the increase in the diameter atthe finish form station, over the entire run, afforded an indication ofsurface deterioration and tool wear. For special purposes, examinationsand tests of other surfaces or cuts, as resulting from the otheroperations, including facing, drilling and reaming, were made. It willbe understood that in instances where tool wear became excessive or themachining operation involved premature destruction of the tool, the testwas terminated short of the total eight hours. Oil coolant was employedin the operations, and chip characteristics were also noted.

These 8-hour screw machine tests, as by comparisons utilizing standardtypes of stainless and other steels, were found to have good correlationwith the experience of several industrial screw machine shops engaged inactual production, because the described test was specially designed toreproduce all the important features of screw machine operation. As willnow be appreciated, the data from each 8-hour test thus represented asignificant formulation of actual production-type machining.

By way of supplement to the screw machine tests, and for a number ofareas where comparative determinations of a simpler nature wereappropriate, various shorter or expedited tests were employed. One suchwas a single point turning test, involving a high speed surface cut(i.e. a helical cut) on a standard piece, for example a 4-inch diameterround bar, utilizing a hardened, high speed steel tool and a continuousrun until destruction of the tool occurred. During the test no cuttingfluid was used. It was found that with surface speeds of 200 surfacefeet per minute and upwards (suitable selection being made for eachtest), one second of tool life in this operation corresponded toapproximately one minute of run of an automatic screw machinefunctioning as outlined above, because high speed tools, i.e. tools ofthe same type were used in both tests. As indicated, these special testsafforded a reliable indication of tool life.

For further introduction to the invention, reference is made to FIGS. 1,2 and 3 of the drawing, being microphotographs at about 500magnification, showing the three generally recognized types of manganesesulfide inclusions in steel. The first are the so-called globularinclusions, being Type I as in FIG. 1. These are round or somewhatelongated bodies predominantly composed of a compound of manganese andsulfur and it has now been demonstrated not only that they are requisitefor providing enhanced machinability by reason of sulfur addition, butalso that they must persist in the final product form of the steel, e.g.after hot rolling or like reduction. In such rolled products theseinclusions, as retained in useful state, are actually of an ellipsoidalshape for the most part, but the term globular will be more generallyemployed herein, being thus understood to include the ellipsoidal orotherwise somewhat elongated forms.

Another type of inclusion, shown in FIG. 2 and identified as Type II,usually consists of much smaller particles, often appearing along grainboundaries or in groups defined by such boundaries, these beingsometimes called eutectic inclusions. In other cases, or particularly asa result of reduction by rolling, these Type II inclusions appear aslong, thin or stringy bodies, but it has been determined that whichevertheir shape, these Type II inclusions are relatively undesirable and donot have the capability of improving machining properties in the mannerof the globular type. It is believed, as indicated by studies, thatwhere circumstances favor the formation of Type I inclusions, the latterare produced by segregation of manganese and sulfur in combination,usually throughout the steel and in approximately uniform though perhapslocally random distribution, at a stage prior to the completion ofsolidification of the cast ingot. Type II inclusions are understood tobe formed or precipitated only on the freezing of that liquid phase orportion of the steel which solidifies last, this stage being very rapidso as not to afford time, as is the case in the formation of theglobular (Type I) bodies, for migration or diffusion of manganesesulfide or its constituents into collected, larger masses.

Type III inclusions are illustrated in FIG. 3, being somewhat large,highly angular bodies of correspondingly irregular shape and oftenhaving a highly irregular distribution in the steel. They seem, at leastin part, to be of the nature of individual crystals or shapes of crystalgrowth. Inclusions of this sort have also been found to be relativelyineffectual, especially because inclusions of this type or form occur,as has now been discovered, when the steel is excessively deoxidizedwith aluminum, with the resultant formation of undesirable Al O As willbe understood, more than one type of inclusion can show up in a givensteel; for example, some Type I inclusions may be formed even though theType II particles are plentiful or greatly predominant, or there may bea number of Type III bodies. Likewise, Type II groups may accompanyinclusions of Type III, e.g. as in FIG. 3. For indication of the generalorder of size of the inclusion bodies, a scale designation of 20 micronshas been added to each of the above microphotographic representations,although it should be understood that the globular inclusions, forinstance, may have a considerable range of dimensions, including sizeswell above those seen in FIG. 1.

In the investigations leading to the present invention, it waseffectively determined that superior results in machining, notably as tosurface finish, tool life and availability of suitably high speeds ofcutting, required a reasonably uniform distribution through the steel ofthe globular-type inclusions, with essentially little or no pressence ofother types. Correlation between machining results and inclusioncharacteristics was found to be very close, and in accordance with theinvention, desired results were further determined to be dependent onsignificant factors, including compositional factors both as to theinclusions and as to the steel, other than the presence of sufficientsulfur and attainment of manganese-sulfur ratios within ranges of thenature heretofore indicated.

Thus for example, in a stainless steel of the 416 grade, containing 13%chromium and other compositional characteristics as prescribed bystandard specifications (including sulfur in the range upwards of 0.15%measurement of machining tool life for various manganese-sulfur ratiosshowed a minimum value to be requisite but also showed that valuessubstantially higher yielded progressively shorter tool life. In thispreliminary investigation, it was noted that a peak of useful tool lifelay between about 2.5 and 4.5 for the stated ratios. A 416 grade steelhaving a manganese-sulfur ratio of only about 1.5 was relatively poor,whereas a heat with the ratio at 3.35 showed much more satisfactoryresults, e.g. a several-fold greater tool life, and surface roughness(in an 8-hour test) at about one-half the value for the steel with thelower ratio. Moreover, microprobe examination revealed a significantchromium content in the inclusions of the low-ratio metal, which wasessentially absent in the higherratio material.

As indicated above, it was found that the inclusion of selenium in thesteel produced an unusually marked improvement in all of the machiningproperties, and was shown to promote and maintain the desireddistribution of globular inclusions, indeed with little or nosensitivity to manganese-sulfur ratios, so long as the manganese wassutfieient to accommodate all of the sulfur present and likewise all ofthe selenium. In general, this was found to involve a ratio betweenmanganese and sulfur plus selenium, of over 2, usually about 2.5 ormore, it being noted that the quantity, as by Weight, of manganese,

9 needed to combine with the selenium is proportionately less than inthe case of sulfur.

Specifically, for both the 416 and 303 grades, containing sulfur upwardsof 0.15%, it was found significant to include selenium in the range ofabout 0.04%, preferably 1 sions according to this invention, ascontrasted with such steel as the standard sulfur-containing 416 typewith manganese content up to and beyond several times the sulfur level.As noted, these comparisons were made between steels produced in theregular manner and with the usual 0.05%, and upwards, but generally lessthan 0.15%, a 5 hot rolling reduction. Conventional practice in makingpreferred range being up to about 0.1%. While in a bar or similar stockincludes extensive hot reduction, broader sense, larger amounts ofselenium can be acthrough the stages of slab and billet to or almost tothe commodated and are conceivably useful, there is special final barsection, the total of such reduction in thickness advantage in utilizingthe lower range which has now being at least 90%, and often more. Theavoidance, in been found to be very effective. That is to say, amountsthe present products, of deterioration due to hot rolling is of seleniumof the order of 0.2% and above are relatively well exhibited (incomparison with the prior sulfur-conuneconomical to add, not onlybecause of the cost of this taining types mentioned above) in allsituations where element but also because losses by vaporization fromthe there has been substantial hot rolling, for instance to 50% meltincrease, proportionately, at a much greater rate than reduction ormore. increases in the concentration which is to be established. a Byway of example, the following represent the com For example, to keep0.1% Se in the steel, it usually positions of typical heats of the 416and 303 types of sufiices to supply about 0.12% (by addition in theladle), stainless steel embodying the invention, the values beingwhereas to reach a level of 0.2% can well require supplyweight percent,as is true for all percentages elsewhere ing more than 0.3% of thiselement, even up to 0.4%, 9 herein except when otherwise specified, andthe balance with still greater proportions of loss at higher levels.being, of course, iron except for incidental impurities:

0 Mn P 8 Si Ni Cr Mo Se A120;

Type No. 416:

Heat A 0.112 1.14 0. 01s 0. 349 0. 39 0. 37 13. 05 0.11 0. 0501 0. 002Heat B-. 0.127 1.08 0.027 0. 290 0. 34 0.33 13.00 0.21 0. 0541.1 0. 002Type No. 303:

Heat A 0.071 1.72 0. 043 0. 224 0. 49 0. 35 17.50 0.65 0. 0501 0. 002Heat B 0.000 1.69 0. 033 0.320 0. 04 9.39 17.25 0. 37 0. 05-01 0. 002

The addition of selenium was specifically noted to pro- Each of theabove was a 70-ton heat, made in an elec vide improvement andstabilization of the desired globular 35 tric furnace in a generallyconventional manner and likeinclusions, to the extent of permittingtheir effective preswise conventionally processed with extensive hotworking, ence regardless of the existence of even relatively high i.e.hot rolling to a high percent of total reduction as manganese-sulfurratios and regardless of other influences explained above, whereby theingot form was converted deleterious to the desired form and shape ofthe inclusions. to the eventual product shape, such as round bars.Excep- An extremely important characteristic of the selenium- 40 tionsto conventional practice included, of course, the sulfur inclusions isthat they substantially retain their glonovel compositionalcharacteristics. Additions were made bular, i.e. ellipsoidal shapethrough conventional forming in appropriate manner, as for example thatmanganese steps (e.g. hot rolling and cold reduction) intermediate wasadded as ferro-manganese. Selenium, conveniently in between the castingof the ingot and ultimate production form of ferro-selenium, was addedto the melt in the ladle, of a bar or the like, this being particularlytrue with reas likewise elemental sulfur in amount necessary to reachspect to hot rolling. Whereas ordinary manganese sulfide a desired totalabove 0.2%, i.e. in the range up to 0.4%, inclusions, even when globularin the as-cast metal, are With some preference for values around 0.3%.The steelsensitive to conversion to a long, thin, stringlyconfiguramaking operation also included specific control of the tion asa result of hot rolling, the sulfur-selenium inclucontent of aluminumoxide, as measured in the final ingot sions of the present inventionhave been formed to retain or billet, with the aid of specialdeoxidation practice in their shape, i.e. by virtue of sufiicienthardness at the rollaccordance with another feature of the invention asexing temperatures of the order of 1,800 F. and higher, so plainedelsewhere herein. that they are still desirably globular in the ultimatebar These steels were found to exhibit superior machining or otherstock. properties, e.g. by the screw machine tests, and to provide,Specifically, it has been discovered that the Mn(S, Se) for example,turning capability with good surface speeds, globular inclusions, heredescribed, are significantly at useful tool life upwards of 8 hours andwith surface harder than Mn(S) inclusions at the hot working temfinisheshaving a roughness below 50 microinches R.M.S., peratures, to thecritical extent that they keep their charsuch speeds being about 200surface feet per minute for acteristics and change only to anellipsoidal or moderately grade 416 and at least about 150 for grade303. elongated form, whereas the Mn(S) inclusions are at least Numeroustests have further shown that the volume in most cases changed by hotrolling to a thin, highly fraction of the globular inclusions in thesteel is impordrawn-out, inferior type, Indeed, it has also been noted,tant, especially for the so-called free-machining grades, on the otherhand, that Mn(S) bodies, even when of suitand, of course, is dependenton the proportions of manable configuration, are relatively harder andcorrespondganese, sulfur and selenium in the compositions. For inglyless appropriate than the present Mn(S, Se) inclueffective realizationof the benefits of the invention, the sions, at the temperatures ofl,00O F. or thereabout, manganese content of the steel (weight percent)should reached locally in the metal by the action of the tool in in mostcases be equal to or greater than the value of machining, 2.5 times(often at least 3 times) the sulfur content, or These facts, includingparticularly the effects of hot advantageously, such value plus that ofthe total content rolling on the inclusions and the correlation betweentype of element or elements of the class consisting of selenium ofinclusion and machinability have been well established and tellurium.Under such circumstances and with suffiby test, and conclusively so byproduction-type machining cient selenium and/or tellurium as elsewhereherein extests as described above. Significant improvement in toolplained, the inclusions or Segregated bodies containing Wear and toollife, surface finish, and other factors inveselements 36 and/or T6 are pminantly and usually tigated under practical working conditions, hasbeen atsubstantially all of globular type, provided by manganese tainedwith the stainless steels having Mn(S, Se) incluin coaction with suchelements, and indeed at least predominantly characterized by thepresence of quantities of such elements in combination with manganese,the inclu sions also being predominantly free of unwanted elements suchas chromium. As also explained herein, tellurium functions similarly toselenium and may in at least a number of instances be employed wholly orpartially as an alternative, although selenium is presently deemed to beof special advantage, economically and otherwise, and is thereforechiefly considered in the exemplification of the invention.

In general, the volume fraction, which is the ratio of the total volumeof inclusions to the total volume of metal, measured in percent, mayrange from 0.1% to an upper convenient limit of about 4%. The optimum orordinarily desired values differ for various grades or types ofstainless steel, and depend on whether the steel is specially designedfor machinability or whether the invention is employed as a supplementalimprovement of machinability in steels where other characteristics areparamount. The obtainable volume fraction of inclusions for a givencontent of S, Se and/ or Te has been measured as lower for austeniticsteel than for martensitic compositions, and the cause of thisdifference, e.g. possibly a result of the microstructure or perhaps aninability to measure the very finest inclusions, is not known, but it isbelieved that in general the content of S, Se and/or Te, at leastsubstantially (e.g. one-half or more) or doubtless predominantly,becomes embodied in useful inclusions.

For the free-machining type 303, which is a chromiumnickel austeniticgrade, a volume fraction of 0.3% to 1.5% has been found very suitable,the range of about 1% and above being especially preferred. In the aboveexamples A and 1B of such steel, the volume fractions were about 0.9 and1.1%. Likewise in the straight-chromium, martensitic, free-machininggrade 416, best results have been achieved with a volume fraction of0.9% to 2.5%, present special preference being for values in the rangeapproaching 2% and upwards. In the stated examples A and B of 416, thevolume fractions were about 1.8 and 1.5 These best and preferred rangesfor austenitic and martensitic grades are believed to be applicable ingeneral to other so-called free-machining stainless steels, respectivelyas some may be of austenitic or martensitic character; more generally,in stainless steels designed to have high machinability, includingferritic grades of the 400 series and precipitation-hardening grades,useful results are achieved in the range of volume fraction, forinclusions, from 0.3 to 4%, preferably 1 to 2% as may be readilyascertained (by test if necessary) for any given composition of suchsteel.

In the case of stainless grades not ordinarily to be classed asfree-machining, of which examples are given hereinbelow and for whichhigh sulfur levels (e.g. even 0.1%) cannot be tolerated in view ofrequirements for corrosion resistance or otherwise, the improvementsherein described are applicable for achieving some useful betterment inmachinability, as at least to aid in some necessary drilling, cutting orshaping operations. In such cases, the volume fraction of the inclusionsmay have to be relatively low, i.e. about 0.1 to 0.2%, but may be higherif possible.

A special requisite of superior machinability in accordance with thepresent invention is the control of the inclusions to have the desiredglobular shape, this being primarily achieved by the statedincorporation of selenium (or tellurium) in the range upwards of 0.01%and for the special machining grades, preferably upwards of 0.04%. Theselenium (or tellurium) content should, moreover, ordinarily be equal inamount to at least one-tenth of the sulfur; a range of one-half toone-eighth has been usefully employed in the special machining typeswith sulfur around 0.2% and above, but providing, of course, that aminimum absolute amount of this element, e.g. selenium, is present.Higher relative proportions of selenium are ordinarily requisite instainless steel grades with very low 12 sulfur, e.g. 0.03% S max.; with0.01 to 0.03% Se, the latter may equal from one-half to twice or more ofthe sulfur content.

Reference is now made to FIGS. 4 and 5 of the drawings, the first ofthese being a microphotographic showing of inclusions of the globular(specifically, ellipsoidal) character which are found in steels of theinvention, such as the 303 and 416 examples noted above. In contrast,FIG. 5 shows long, thin inclusions which, as explained above, areundesirable and which are found, for example, in a 416 grade steellacking selenium and processed, through hot rolling, from a heat havinga manganese-sulfur ratio of about 3. Examination of a number of heatsmade in accordance with the invention have indicated that the inclusions(in the final product, after hot working) are at least predominantly,preferably very predominantly, Type I as shown in FIG. 4, it beingunderstood that advantageously or more of the total inclusion volume,and most usefully over is in this form. The long thin inclusions, ofprior products, such as in FIG. 5, appear to have a ratio of length todiameter of more than 10, often well over 10, whereas such ratio for theglobular type of bodies is usually substantially smaller, beingpredominantly (and preferably nearly all) 5 or less, with notably goodresults where their length-to-diameter ratios are predominantly no morethan 3, or even 2.

Turning now to FIGS. 6 and 7, these bar graphs afford some informationabout the examples of 303 and 416 grades made in accordance with theinvention, e.g. as above. These are computer-plotted graphs ofinformation derived, by sensitive scanning instrument, from sectionsthrough the steels, and represent the size distribution of transverseand longitudinal dimensions of the measured intercepts of theinclusions. Since the intercepts or sections of the ellipsoidalinclusions will often or perhaps mostly occur at other than centrallocalities, the measured dimensions have average values considerablysmaller than those of the actual inclusions, which are believed topredominate, roughly, in a length range of 2 to 20 or 30 microns (withinstances up to 50 or even 80), but the plotted data are deemed of somesignificance in a relative sense, e.g. for comparison with readings ofother specimens made On intercepts in the same way. Moreover, the graphsexclude measurements less than 0.2 micron, as being below the reliablerange of the measuring instrument, but it is likely that the curveswould slope down to low values in such regions.

In each graph, the broken lines represent the transverse (narrower)dimensions and the solid lines, i.e. horizontal, represent thelongitudinal dimensions or length of the inclusion intercepts. Thus forexample in FIG. 6 (grade 416) about 45% of the measuredinclusion-section widths (transverse) were between 0.4 and 0.8 micron,and about 33% of the measured lengths were in the same range; otherparticulars of size distribution for the intercepts are similarlyreadable in FIG. 6, and likewise in FIG. 7 for one example of grade 303.As stated, all this relates to the size of the inclusion sections thatwere intercepted, rather than to the actual inclusion sizes, which weremuch larger.

A further feature of the invention, embodied in the examples ofstainless steel set forth above, and in their method of production,embraces the control of such production, including special aspects ofthe treatment of the metal, so as to afford an output of produced steelwherein the content of aluminum oxide is reduced to and maintained at anextremely low value. In preliminary examination of free-machinin steels,for example of the 416 grade, it was noted upon certain tool life teststhat a marked difference in tool life existed between various heats,despite little or no compositional variation or processing difference ofordinarily recognized sort. It was discovered, upon extended furthertests, including accurate chemical analysis of the steels for alumina(determined as acid-insoluble aluminum), that such aluminum oxide was asignificant factor in tool wear and tool life. Indeed, despite the factthat instead of killing (deoxidizing) the steel with the usual additionof aluminum, tests were run Where the steel (otherwise embodying theinvention, as to inclusions) was deoxidized with silicon (supplied asferrosilicon), considerable difiiculty still persisted. When drasticeffort, however, was thereafter made to avoid the addition of anyappreciable aluminum in the killing step, specificcally by usingferrosilicon of extremely low aluminum content, a very markedimprovement in tool life could be achieved quite consistently in theproduced steel.

FIG. 11 shows the results of 8-hour screw machine tests on specimensfrom two heats of steel which are respectively designated as C and D,both being stainless grades of type 416 having manganese sulfideinclusions and manganese-sulfur ratios respectively of 2.9 and 3.2, i.e.within the range presumably requisite for useful machining. Thecompositions were, approximately, 0.13% C, 1.1% Mn, 13% Cr, otherelements within A.I.S.I. maximum limits for this grade, S respectively0.38 and 0.33%, and no Se or Se. In FIG. 11, the surface finish isplotted against the produced number of pieces, for these two steelspecimens. Heat C maintained a fairly level surface finish throughoutabout 1,500 pieces at 180 s.f.p.m. (surface feet per minute) whereas theroughness of the machined surfaces from the bars of heat D rose to avery high value at 175 pieces at 162 s.f.p.m., indeed virtuallydestroying the tool. n chemical examination, heat C contained only0.0005 aluminum oxide, while heat D was analyzed to have 0.0055%. Inspecial single-point-turning, dry tool life tests, designedlyabbreviated by using no cutting fluid, like results of markedly longertool life on the low-alumina metal C were obtained, by a factor ofseveral times the tool life on metal D.

Further significant results are shown in FIG. 12 where a series of heatsof 416 type stainless steel having the same kind of basic composition asheats C and D are shown as subjected to the automatic screw machinetest, being respectively as follows:

1. A heat herein designated E, having a manganesesulfur ratio of 2.92,but containing no selenium addition and thus in no way embodying thepresent invention. In the production of this heat it was killed withordinary ferrosilicon (75% Si, 1 to 2% Al), no effort being made toavoid aluminum as an impurity in the latter. The steel analyzed 0.004%A1 0 2. In a heat designated F, selenium was added, in amount of 0.05%,the manganese-sulfur ratio being still approximately 3, i.e. 3.12. Thecontent of A1 0 was 0.0038%.

3. A heat G, wherein selenium was also added in the same proportion, andthe manganese-sulfur ratio was again about the same, being 2.92.However, in this instance the deoxidation was effected by adding aspecial grade of ferrosilicon containing very little aluminum, i.e. 75%Si, 0.4% A1 max. Analysis showed only 0.0006% A1 0 As will be seen atonce from FIG. 12, with the test run at a cutting speed of approximately200 s.f.p.m. in all cases and cross slide feed of about of an end slidefeed of 0.0045 inches per revolution, there was essentially zero life ofthe finish forming tool for the steel of heat E. The surface roughnessrose immediately to 140 microinches R.M.S. and the test was promptlyinterrupted. Although presumably a similar aluminum impurity occurred inthe steel of heat F., useful machining was obtained through a run ofover 1400 pieces, with surface finish con sistently under 100 microinchroughness. Finally, with the very low aluminum content in thedeoxidizing addition, heat G, unusual machining properties wereobtained. A full run of 1,700 pieces was performed, maintaining surfacefinish with roughness well under 40 microinches R.M.S. throughout.

As will be noted, the provision of globular Mn(S, Se) inclusions in thetested steel bar stock of heats F and G by virtue of the seleniumaddition improved the machining properties in very marked degree, bothas to tool life and surface finish. On comparison of the results forheats F. and F with FIG. 11, it is also apparent that the improvedcomposition and inclusion structure of heat F had substantial effect incounteracting or toward overcoming the adverse influence of the aluminumoxide particles. The test with the final heat, G, demonstratedimpressively the result of reduction of alumina to a very low value, andalso the improved, overall machining properties achieved by the specificsulfur-selenium type inclusions.

These results are confirmed by the microphotographic views of FIGS. 8, 9and 10, FIG. 8 showing a steel such as heat E, with relatively imperfectsulfide inclusions, and large particles of aluminum oxide, being thevery dark irregular masses. In FIG. 10, which shows a steel containingselenium but with no effort to reduce aluminum oxidethus correspondingto heat F-the dark aluminum oxide particles are noted to have becomeincorporated with the sulfide-selenium inclusions, and indeed in part tobe coated or covered by the material of such inclusions. Finally FIG. 9shows the highly desirable, alumina-free inclusions constituted by steelsuch as that of heat G or the specific examples of 416 and 303 (A and Bfor each) given above.

In general, it is found that the aluminum oxide in the finished billetof steel should be not more than 0.0025 and indeed most advantageouslyand critically for best results, not more than 0.002%. Experience hasalso indicated that where ferrosilicon, containing 75% silicon byweight, is added for deoxidation, usually in amounts between 5 and 10lbs. per ton, the aluminum content of this material, e.g. as impurity init, should not exceed 0.5%, and preferably lower, even down to 0.1% ifpossible. The actual amount of ferrosilicon added for a given heat will,of course, depend on the amount of silicon already present, whetherincidentally or otherwise, and available to coact with the ladleaddition. As indicated below, other agents can be used instead ofsilicon. While in theory vacuum deoxidation should be appropriate, ithas appeared to involve some difficulty because the preferred Mn(S, Se)inclusions apparently embrace some oxy-type combination of the elementsfor best effect and vacuum treatment depletes the available oxygen toomuch. It is nevertheless conceived that in some cases and with specialcontrol or other compensation, vacuum techniques may not necessarily beexcluded.

It is particularly noted that although the total aluminum content of thesteel should preferably be kept as low as possible, and indeedordinarily at a level no greater than what would be considered asincidental impurity, the critical condition is related explicitly toaluminium oxide, conveniently analyzed as acid-insoluble aluminum andreported or calculated as the oxide. Indeed, it has appeared thatstainless steels produced in accordance with the present invention andhaving alumina well below 0.002% may nevertheless have a total aluminumcontent somewhat higher than that accounted for by alumina inclusions,such excess aluminum being presumably alloyed in the steel matrix. Inother words, small quantities of aluminum presumably present 'as metaland other acidsoluble form may be tolerated perhaps because they aredissolved in the melt in the beginning and do not participate indeoxidation reactions to the extent of aluminum added in the furnace orlater, but in any even it is important to minimize aluminum additions,even incidentally, which have opportunity for conversion to oxide.

By way of further evidence of the aluminum oxide effect, -a group of 12heats of 416 grade stainless steel, which contained selenium and thusinvolved the improved inclusion structure, and which were subjected todeoxidation with ferrosilicon of low aluminum content, were subjected toanalysis for aluminium oxide and were also, in appropriate bar form,subjected to expedited tool life tests. The stainless steel of 9 ofthese heats showed aluminum oxide content ranging from 0.0009% to0.0018% and afforded tool life by the above tests in the range from 15170 to 418 seconds. In contrast, 3 of the heats showed alumina analysisof 0.0028 to 0.0031%, and a lower range of tool life, namely 77 to 116seconds. The advantages of very low alumina content were thus furtherdemonstrated, as Well as the importance of critical control to assureproduction of the desired low-alumina metal.

A particularly. effective procedure for producing stainless steel inaccordance with the invent-ion thus includes the steps of deoxidizingthe metal, as in the ladle, by addition of silicon or other agent havingno more than a very low aluminum content. In the case of ferrosilicon,containing 75% Si, this should be not more than 0.5% aluminum. Moregenerally stated, it appears that the oxidizing agent, of which otherexamples are rare earth elements such as lanthanum, cerium, and others,should not introduce more than about 0.003% aluminum, measured as weightpercent of the steel, and preferably not more than 0.0025%. A furtherstep in the production process is that each completed heat of steel istested by analysis, for example in ingot or billet form, to determinethe aluminum oxide content, and the actual production of finished metalto constitute a truly machinable product is selected as those heats forwhich the analysis shows a content of alumina not greater than 0.002%.Thus where for some indeterminable reason an occasional heat may reveala significantly higher alumina concentration, the product may bediverted to other uses, 50 that the controlling operation, as justexplained, restricts production to the stated limit.

Analysis of the aluminum oxide present may be achieved in any suitablefashion, i.e. in accordance with any of various available chemical andspectrographic procedures. One suitable mode of examination, forinstance, has involved obtaining a quantity of chips of the steel,including fine particles, by milling or drilling, e.g. 10-15 grams.These are dissolved in suitable acid (hydrochloric and hydrofluoric) andfiltered. The residue containing the acid insoluble aluminum may then beanalyzed for such aluminum by appropriate spectographic technique. Forinstance, one convenient process involving fusing the residue inpotassium pyrosulfate, and then dissolving the solidified fusion productin concentrated hydrochloric acid containing yttrium (dissolved thereinas oxide) as internal spectrographic standard. This solution was thenutilized for analysis by emission spectrography with a rotating diskspectrograph, the amount of aluminum being determined by a densitometerreading of the exposed and developed plate from the instrument. Asstated, it is understood that chemical and like procedures suitable fordetermination of acid-insoluble aluminum, e.g. as aluminum oxide, are ineffect known, although not heretofore routinely employed in steel makingpractice.

As also indicated, special advantage in machining operations, notably asto chip characteristics, was realized by subjecting the martensiticgrade steels to a special heat treatment. More specifically, instead ofthe usual solution treatment of the hot-rolled product in the range of1,800 to 1850 'F., followed by the usual quenching or air cooling andthereafter tempering, specimens of the 416 grade stainless steel (as ofthe composition in the examples above) were normalized by heating in themultiphase re- :gion between the upper and lower critical temperatures.This involved a heat treatment at normalizing temperatures, for steelsof about 13% chromium, about 1.1 to 1.2% manganese, about 0.3 sulfur,and 0.05% to 0.1% selenium, which were in the range of 1600 to 1700 F.The steel was held at this normalizing temperature, e.g. 1700 F., forabout 1 /2 hours for one inch diameter bars, then air cooled to roomtemperature or at least below 200 F., and thereafter tempered inconventional manner to a desired hardness.

Samples of various heats of steel treated in this manner showedconsiderable improvement in chip characteristics on machining, forexample in tests of drilling utilizing one half 1 5 1 9. .9116 inchdiameter drills at speeds of 300 r.p.m. or above, with appropriatelubrication. Instead of long, ribbon-like chips, sometimes several feetin length the chips tended to break off in much shorter and moremanageable fashion.

A preferred treatment, which is believed to result in a microstructureof martensite and ferrite that is peculiarly appropriate for machining,involved first heating the steel, in rod or other finished stock shapeafter all hot rolling and any cold drawing that may have been used, tothe lower critical temperature (which might be, for example, 1500 F),and holding at that temperature for one hour or more. The temperaturewas then raised about F. and again held for a predetermined time, forexample one hour or more, being thereafter air cooled to a suitable lowvalue, such as room temperature, or more generally, below 200 F.

Again, the steel was tempered in conventional fashion, with attainmentof hardness in the range commonly desired for martensitic stainlesssteel of these grades. The tempering treatment involved heating attemperatures conventionally appropriate, for instance in the range 1050to 1100 F. for 1%. hours, then cooling to room temperature. There was noditficulty in attaining desired hardness by selection of temperingconditions in conventional manner, e.g. Brinell hardness values in therange of to 250, preferably 195 to 220; nor was there difficulty inultimately hardening machined products by standard procedure tosatisfactory values such as Rockwell 40C to 45C.

Specific test results with this preferred treatment of type 416 steel,compositionally conforming to the present invention and produced to havethe above-stated low content of aluminum oxide, showed even furtherimprovement in chip characteristics on drilling tests, for a widevariety of compositions (whereas the simple treatment was not assatisfactory for higher-manganese metal, e.g. over 1.5%, as on low-Mnsteel) While other characteristics of machinability remained entirelysatisfactory, i.e. at the levels of superiority described above.

By way of example, the following table sets forth the compositions of anumber of steels which were subjected to the preferred type of heattreatment and which also serve to illustrate further compositionalvariations in this 416 grade, within the invention:

Drilling tests were performed on these steels in the above-describedmanner with one-half inch diameter drills operating under oil flood at aspeed of 315 r.p.m. and a feed of 0.00515 inch per revolution, and alsowith one inch drills at lower speed and feed. In all cases the drillchips were considered good to excellent, being tightly curled, brittlepieces, for the most part relatively short. This was in distinction tothe experience of like tests with steels of similar composition that hadnot received the special heat treatment. In the latter cases, the chipstended to be tough, whether long or short, and likewise to be open oralmost straight, with very long chips tending to predominate.

It will also be understood that the upper and lower critical points varywith the composition of the steel, in accordance with recognizedprinciples, determinatons for particular cases being thus readilyachieved from known data, or by tests if necessary. Thus the criticalpoint values vary with composition, particularly manganese content inthese martensitic grades of the 400 series. Both the lower and uppercritical points fall with increase of manganese, the change in the lowercritical temperature being considerably larger in proportion. Forexample, in 13% chromium stainless steels, the lower critical point (Ais in the range of 1560 to 1425 F. for 0.45 to 17 2.14% Mn, and theupper point (A or complete austenitizing temperature, is in the range of1775 to 1750 F. for the same Mn range.

As will be understood, the critical points, which define the range overwhich the structure of the steel undergoes change in the usual manner(being completely austenite above the upper point), are difierent forheating and cooling, being higher when attained in the course ofheating. The critical points mentioned and exemplified above are thosefor heating, and for brevity these point are simply identified as A andA without a further qualifying des ignation.

In preferred practice of the present invention, as relating tomartensitic stainless steels characterized by the defined Mn (S, Se)inclusions, the basic or simpler heat treatment involves a temperaturewell within the A -A region, e.g. at least 50 F. above A, and 50 F.below A and advantageously in a range departing by about 100 F. fromeach point, and where the two-stage operation is used, starting at A thesecond step is preferably 100 F. to 150 F. above it. The time at eachselected temperature, for either mode, is usually one hour or more,preferably 1 /2 hours for bars and the like, and longer times forheavier sections.

Advantage has been achieved by addition, to compositions embodying theimproved Mn(S, Se) inclusion, of rare earth elements, in general any oneor more of this known class and in a specific, practical sense,combinations of lanthanum and cerium or a selection of one or more ofthe so-called cerium earths, notably lanthanum, cerium, and neodymium.Not only do such additions tend to promote formation of Type I sulfideinclusions at the expense of other types, but tests have revealedspecific improvement in machinability for stainless steel compositionsotherwise conforming to the invention. For example, selected ingots ofthe 416 heat designated H in the last previous table above, weresubjected to addition (in the molten condition of the steel) ofquantities of a commercial rare earth alloy called Lancelloy andconsisting principally of metallic lanthanum and metallic cerium. Theresulting steel products, along with steel from an untreated ingot, weresubjected to machinability tests (after the usual hot rollingreduction), specifically a single point turning test as described aboveat 200 s.f.p.m., yielding tool life determinations in seconds.

As will be noted, the rare earth metal additions in this selected caseafforded an improvement in tool life, of notable advantage especially inthat tool life without the additions happened to be somewhat less thanoptimum. Separate tests indicated that this heat had excellent machinedsurface finish characteristics, which were not significantly afiected bythe rare earth additions. Some tests on steel of another specificcomposition tended to indicate that machinability improvement with rareearth metals may involve some correlation between the amount of suchaddition and the content of sulfur, or sulfur and selenium, in thesteel. For instance, with a lower sulfur content than in the above heat,tool life improvement was selectively noted for an addition of 2 lbs.rather than 3 lbs. of the rare earth alloy per ton. There was alsoindication, in further tests, that with a larger content of manganese,machinability advantage with rare earth elements may be less.

It was further noted from microprobe examinations of certain of thesesteels that some inclusions appeared, or parts of the sulfide-selenideinclusions, where lanthanum and cerium tended to concentrate inassociation with silicon and oxygen to the exclusion of manganese andsulfur. Inclusion bodies of this type presumably embraced oxides oroxygen-containing compounds of the rare earths, but represented only avery minor fraction of the total influsion volume and at least for suchreason appeared not to affect the machining properties adversely. Ingeneral, the rare earth additions tended to be beneficial, especially inheats with manganese content below, for example, 1.4%.

A notable utility of the rare earth additions is that they may serve toeffect deoxidation, e.g. in lieu of other agents such as silicon or incombination with the latter. Thus the procedure of making stainlesssteel, of any of the various grades contemplated by the invention, mayinclude the step of supplying rare earth metals, in suitable metallicform as above, to the melt at the time of pouring, for instance in theladle in appropriate amount, as of the order of 2 to 4 lbs. per ton. Forthe beneficial effect of lanthanum, cerium or the like in theinclusions, the present indication is that the rare earth content is inthe range of "0.02 to 0.3%, preferably 0.05 to 0.2%, the above additionsto heat H, measured as 2 and 3 pounds per ton, being equivalent to about0.1 and 0.15%, respectively.

Although the several features of the invention have been chieflyexemplified above with respect to the martensitic grade 416, they havebeen demonstrated to be effective in other grades to which they areapplicable.

Thus the improved nature of the Mn(S, Se) inclusions has been achievedin the austenitic grade 303, and likewise the controlled limitation ofaluminum oxide to very low values, the procedure and resultingcompositions, being essentially identical in each case for this otherfreemachining type, namely as to content of S and Se and as tomaintenance of alumina below 0.002%, preferably well below. All of thishas been demonstrated with excellent results on the 8-hour screw machinetests, in a number of other 70-ton heats of type 303, additionally tothose designated A and B above. In these heats, the compositions rangedapproximately as follows: 0.07 to 0.12% C (mostly below 0.17), 1.6 to1.9% Mn, 0.025 to 0.04% P, 0.26 to 0.35% S (mostly above 0.3%, 0.3 to0.7% Si, 0.15 to 0.35% Cu, 9.05 to 9.5% Ni, 17.0 to 18.2% Cr, 0.22 to0.58% Mo, and 0.04 to 0.08% Se, with A1 0 below 0.002% in the severalinstances Where it was controlled.

It has also been demonstrated, by a number of examples, that otherstainless steels are susceptible of improvement in machining propertiesin accordance with the principles of the invention. In the case ofgrades heretofore considered to be free-machining, such as A.I.S.I.430F, which is a ferritic steel of straightchromium type with chromium14-48% (usually about 17%), the composition as to sulfur, selenium ortellurium and low content of aluminum oxide may be the same as formartensitic grade 416. Some tests of Type 430-F compositionally modifiedin this manner have indicated that the desired inclusions were presentand have shown marked improvement in various aspects of machinability,comparable to results with Type 416. Where other grades of the 400series have previously been designed to be machinable, as with sulfuradditions, it is conceived that similar compositional features areappropriate, examples of such grades being 420F (like 416, but withhigher carbon) and 440F (chromium 16l8%, carbon about 1%), these beingboth martensitic and also susceptible of improvement by the special heattreatment described above.

In other cases, it is sometimes desirable to improve machinability, eventhough the ultimate uses of the steel do not permit the magnitude ofsulfur and other additions which would afford machining properties thatapproach grades such as 303 and 416. Thus for example the followingrepresent analyses of heats of grades 304 and 316 to which were added anamount of selenium designed to afiord Mn(S, Se) inclusions of thedesired type as described above, in coaction with the low amount ofsulfur tolerated in such steels:

Machinability tests of these heats designated L and M showed that theproperties were appreciably improved over those of ordinary heats of thestandard compositions. The aluminum oxide content was, in each case,controlled to fall below 0.002%, but since these are austenitic steels,the special heat treatment was not employed. As will be understood,these grades are intended to have high corrosion resistance (very high,in the case of 316), thus limiting the amount of sulfur that might beincluded. Type 304 is also expected to be capable of highly polished orbright surface characteristics for decorative purposes.

It should be noted that in the situation of these and other types wherethe level of sulfur and selenium is relatively low and the volume ratioof inclusions is correspondingly low, e.g. from 0.1 to 0.5%, vacuumdeoxidation techniques have been indicated to be suitable. To someextent in very low sulfur grades, such as the basic 12-chrome grade 410,it may be permitted to increase the sulfur content somewhat, inaccompaniment to the Se additions, e.g. possibly to 0.05% S with 0.02 to0.03% or more Se.

Improvement has also been noted, by test, for grade 203 (17% chromium,6% nickel, 6% manganese) with compositional characteristics inaccordance with the invention. Indeed with sulfur content in this typeof stainless steel at levels of 0.2 to 0.35 selenium additions in therange of 0.02 and preferably upward have been effected, yielding thedesired type of inclusions and a volume fraction of inclusions, forexample 0.5 to 1%, approaching the situation of the improved 303 grade,with corresponding enhancement of machinability.

Still another type of stainless steel for which some tests of theinvention have been made is the so-called 17-4 precipitation hardenablesteel, as for example 0.045% C, 3.4% Cu, 4% Ni, 16% Cr. In this instancelimited addition of selenium, where sulfur content was around 0.01 to0.02%, afforded modest improvement, while in situations where the sulfurcontent was allowed to rise to 0.15% and above, correspondingly largeramounts of selenium, e.g. 0.03 to 0.1% were employed with significantadvantage in the machining properties. In all of these cases it will beunderstood that the control of alumina to low values is readilyapplicable with corresponding advantage. Such feature cannot, of course,be employed in special types which require a significant aluminumcontent, such as grade 405 and the semi-austenitic grades ofprecipitation hardenable steels. In the case of the stainless grades ofmartensitic character that are to be precipitation hardened, such as17-4 and -5, the special heat treatment can be employed, i.e. insubstitution for a conventional treatment, without substantial detrimentto the ultimate precipitation hardening which thereafter involvesheating at 900 F. or higher, and air quenching, and which is performedon the finished piece after all machining and forming.

FIG. 13 shows the results of an 8-hour screw machine test for stainlesssteel of the 416 grade, utilizing tellurium instead of selenium. Thecomposition was essentially similar to several 416 heats above, havingabout 0.11% C, 1.1% Mn, 12.6% Cr, and with sulfur about 0.35%, Te 0.04%,and no Se. The uppermost curve N in the figure is that for a completetest with steel from an ingot of such heat which did not have the Teaddition, while the lower curve P represents an ingot in which thetellurium addition was made, both tests being performed at 180 s.f.p.m.for a full run of 1,500 pieces. The comparison pieces N showed arelatively high surface roughness, rising early in the run to a ratherhigh value (in RMS microinches), also indicating a considerable toolwear. In contrast, Tecontaining specimens were found to machine to asignificantly smoother surface finish (roughness less, of the order ofone-half), e.g. as indicated by curve P. It appears that tellurium canbe used instead of Se, for like effect and in essentially the sameamounts, although (as indicated above) some special advantage has beenindicated for selenium, including the fact that with Te additions of0.04% and upwards the steel is likely to require higher hot rollingtemperature, 2,000" F. and above.

In carrying out the invention, usual steel making practices can befollowed, as may be appropriate for the selected stainless grade,including conventional electric or other furnace techniques andconventional modes of incorporating the usual ingredients and thereafterpouring ingots and reducing the steel by hot rolling or other hotworking to the desired final shape. In the case of round bar and similarproducts, cold drawing may be performed as final stage affording anultimate, small percentage of reduction, for the usual reasons. In allcases, the compositions are modified as described herein including theaddition of selenium or tellurium to the melt in the ladle or ingotmold, conveniently as a ferro-alloy usually containing about 50% of thedesired element. Control is advantageously exerted over the aluminumoxide content in the manner described, including use of appropriate,special deoxidation procedure. Finally, for the martensitic grades thespecial heat treatment is preferably performed in an inert furnaceatmosphere, for example of a sort suitable for other heat treatments ofstainless steel.

The results of the invention in respect to machinability are unusuallygood, especially in grades such as 303 and 416. As will be understood,the inclusions appear to function very effectively, and indeed appear tosatisfy very well two specific aspects of their function, namely that inmachining operations the inclusions produce microcracks in the shearzone, thus promoting local fracture and reducing the energy consumed,and further, that the inclusion material deposits on the tool surface,in very small amounts, thus reducing tool wear. The attainment of theseresults as to machinability has been thoroughly established with the8-hour screw machine tests and indeed with such tests of the severalmajor features, notably in the case of grades 303 and 416, as embodiedor carried out in largescale heats, e.g. regular 70-ton electric furnaceheats.

It has been specifically found that whereas preliminary and sometimesconfirming tests with laboratory size heats, i.e. of the order of lbs.,are necessary and advantageous, information about inclusions of thissort in laboratory heats is apt to be misleading or inconclusive. Withvery small ingots, cooling effects, solution effects, dendrite spacing,convection currents and other factors related to solidification are aptto be quite different from large ingots of the order of 2 tons or more,which cool very slowly. In this connection, it must be remembered thatthese inclusions are formed and their characteristics as to shape andnature are determined during and at the end of solidification so thatinclusion structure or morphology is essentially only predictable foractual production heats by making tests with heats of such magnitude.

In a broad sense, stainless steels here contemplated include such as maycontain 10-27% chromium and 0 to 22% nickel. The so-calledstraight-chromium grades (e.g. up to 27% Cr) usually have less than 3%Ni or in most cases substantially less than 1%, e.g. as in type 416,WhlChl has 12-14% Cr. In general, the chromium-nickel grades. maycontain 14-26% Cr and 4-22% Ni with many types, among the 300 series,characterized by 15-21% Cr and 6-15% Ni, the nickel content being 8-13%for certain more common austenitic types. Thus grade 303 is specified as17-19% Cr and 8-10% Ni.

While manganese can range from 0.3 to 10% in stainless steels, apreferred minimum for the invem'tion is 0.8%, with special advantage, inthe free-machirfirg grades, at 1% or more and in some instances,notably'the. austenitic series, 1.5% or above; not more than 3% isnecessary in many cases, and indeed preferably not more than 2%, oradvantageously less.

Optional or incidental elements in stainless steels (conceived to betolerable in broader applications of the invention) may include up to 4%molybdenum, though usually below 1% in the machining grades, up to 5%copper when desired, and up to 1.5 silicon, but preferably not over 1%Si. Total additions of minor, special-purpose elements up to 2% (e.g. upto 1.5 of any one) are conceivable, for instance such as Ti, Cb, Ta, Coand Zr. In all cases, of course, the balance of the composition is iron(e.g. at least 50%) and incidental impurities, together with carbon 0.01to 1.2%, more usually 0.05 to 0.2%. All percentages herein are byweight, except in reference to the volume content of inclusions.

While sulfur can range from 0.01 to 0.7%, it is more often at least0.02% and advantageously not above 0.5%. For machining grades a minimumis 0.15%, but for best results with the invention, at least 0.2% andnotably 0.3%, e.g. in the range to 0.45% or conveniently not more than0.4%. While in a broad sense the material of the selenium and telluriumclass can range up to 0.3%, or with further cost, to 0.4%, there isspecial advantage in the economical lower ranges noted earlier above.Indeed some drill penetration-time tests have indicated little, if any,advantage in that specific respect, in carrying selenium to as much as0.15%, or indeed much over 0.1%. The content of this addition ispreferably 0.02% or above, or advantageously at least 0.03%, to approachspecial machinability as evidenced by volume fraction of inclusions.Such inclusion volume content is advantageously 0.2% and preferably0.3%, or more, a minimum of 0 .5% by volume being greatly preferred toachieve a machining-type steel. Maintenance of an element such asselenium at the lowest weight-percent level consistent with optimumresults is specially desirable, in that such element, in excess, maytend to have a harmful effect on surface properties of stainless steel.

Especially in preferred embodiments, the invention affords notableimprovement in machinability, attributed in significant part to thecontent of relatively large globular inclusions, which are substantiallyfree of iron and chromium and which are understood to consistessentially, or at least predominantly, of the nature of sulfides,selenides and tellurides of manganese, such terms being employed toinclude so-called oxy compounds, e.g. oxysulfide. It appears, forexample, that Mn (S, Se) inclusions have a higher melting point than MnSbodies, and thus can form properly before complete solidification of thesteel, and indeed selenium, of itself, appears to form only globulartype inclusions.

The practical results have been abundantly demonstrated by the screwmachine tests, where the automatic machine runs continuously for 8hours, with the cutting tools repeatedly used in conventional manner,i.e. in the automatically repeated sets of machining operations. Verysatisfactory tool life (8-hours or more of such machine run) andexcellent surface finish are attained for the machining grades of thesesteels, at surface speeds up to at least 200 feet per minute. Whereassurface finish is recognized as conventionally better at higher speeds,the great difficulty, that has now been overcome, has been thatexcessive tool wear and relatively short tool life have heretoforeusually prevented the attainment of such speeds in machining stainlesssteel.

I claim:

1. A method of making stainless steel of a martensitic grade to haveimproved machinability, comprising establishing a solidified stainlesssteel which is compositionally capable of transformation to martensiteand which contains globular inclusions that are distributed therein andthat comprise Mn, S and element or elements selected from the classconsisting of selenium and tellurium, converting said steel to a shapedobject, and thereafter treating said shaped object to characterize thesteel thereof as capable of yielding short-breaking chips on machining,by heating the object to a temperature between the lower and uppercritical points of the steel, for a period of at least one hour, andthereupon cooling the object.

2. A method as defined in claim 1, in which said treating step includesfirst heating the object to approximately the lower critical point ofthe steel for at least one hour, and thereafter raising the temperatureto effect said heating between the critical points.

3. A method as defined in claim 2, in which said raising of temperatureis effected by about 100 F. and said heating between the critical pointsis effected at said lastmentioned, raised temperature.

4. A method as defined in claim 1, in which the steel has a content ofMn, S and said selected element or elements which provides said globularinclusions, in said shaped object, consisting predominantly of Mn, S andsaid selected element or elements, and constituting at least 1% byvolume of the steel, said conversion of the steel to said shaped objectincluding hot rolling to a thickness reduction of at least about 50%.

5. A method as defined in claim 4, in which said treating step includesfirst heating the object to approximately the lower critical point ofthe steel for at least one hour and thereafter effecting the aforesaidheating at the temperature between the critical points by raising thetemperature of the object by at least about 100 F. and holding theobject at said last-mentioned, raised temperature for at least one hour.

6. A method as defined in claim 5, in which the selected element is Seand said stainless steel is a straightchromium grade containing 12 to14% Cr.

References Cited UNITED STATES PATENTS 3,301,663 1/1967 Wicher -573,459,540 8/1969 Tisdale 75-129 3,467,167 9/ 1969 Mahin 7557 3,723,0943/1973 Schlatter 7553 3,314,782 4/1967 Arnaud 75-57 3,235,415 2/1966Palty 148135 2,576,782 11/1951 Daley l48-135 2,799,039 9/1961 Lulal48--135 L. DEWAYNE RUTLEDGE, [Primary Examiner P. D. ROSENBERG,Assistant Examiner U.S. c1. X.R.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION Page1 of 2 PATENT NO. I 3,846,189

DATED November 5, 1974 lN\/ ENTOR(S) I VIJAYAKUMAR A. TIPNIS It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1, lines 4 and 5 (Title Section), after "Vijayakumar A. Tipnis,10260 Pleasant Lake Blvd., Parma, Ohio 44l30"insert assignor to RepublicSteel Corporation, Cleveland, Ohio Column 1, line 24, "aluminium" shouldread aluminum Column 3, line 70, delete after 430F Column 5, line 1,"maner" should read manner Column 9, line 47, "stringly" should readstringy a Column 13, line 21, "Se" (second occurrence) should readColumn 13, line 55, 0.4% should read 0.5%

Column 14, line 49, "aluminium" should read aluminum Column 14, line 62,"even" should read event Column 15, line 39, spectographic" should readspectrographic Column 15, line 65, after "0.3" insert Column 17, line10, "point" should read points Column 18, line 4, "influ-" should readinclu- Column 18, line 39, "0.17" should read 0.1%

Page 2 of 2 UNITED STATES PATENT OFFICE Patent No. 3,846, 189 DatedNovember 5, 197A Inventor(s) vijayakumar Tipnis It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 22, line 56 (References Cited Section), "2,799,039"

should read 2,999,039

gigncd and Sealed this Twenty-first Day of February I978 [SEAL] Attest:

RUTH C. MASON LUTRELLE F. PARKER vl lttesting Officer ActingCommissioner of Patents and Trademarks

1. A METHOD OF MAKING STAINLESS STEEL OF A MARTENSITIC GRADE TO HAVEIMPROVED MACHINABILITY, COMPRISING ESTABILISHING A SOLIDIFIED STAINLESSSTEEL WHICH IS COMPOSITIONALLY CAPABLE OF TRANSFORMATION TO MARTENSITEAND WHICH CONTAINS GLOBULAR INCLUSIONS THAT ARE DISTRIBUTED THEREIN ANDTHAT COMPRISE MN, S AND ELEMENT OR ELEMENTS SELECTED FROM THE CLASSCONSISTING OF SELENIUM AND TELLURIUM, CONVERTING SAID STEEL TO A SHAPEDOBJECT, AND THEREAFTER TREATING SAID SHAPED OBJECT TO CHARACTERIZE THESTEEL THEREOF AS CAPABLE OF YIELDING SHORT-BREAKING CHIPS ON MACHINING,BY HEATING THE OBJECT TO A TEMPERATURE BETWEEN THE LOWER AND UPPERCRITICAL POINTS OF THE STEEL, FOR A PERIOD OF AT LEAST ONE HOUR, ANDTHEREUPON COOLING THE OBJECT.